Light valves have been known for more than sixty years for modulation of light. They have been proposed for use in numerous applications during that time including, e.g., alphanumeric displays and television displays, filters for lamps, cameras, optical fibers and displays, and windows, sunroofs, sunvisors, eyeglasses, goggles, mirrors and the like to control the amount of light passing therethrough or reflected therefrom as the case may be. Examples of windows, without limitation, include architectural windows for commercial buildings, greenhouses and residences, windows for automotive vehicles, boats, trains, planes and spacecraft, windows for doors including peepholes, and windows for appliances such as ovens and refrigerators including compartments thereof. Light valves of the type described herein are also known as “suspended particle devices” or “SPDs”.
As used herein, the term “light valve” describes a cell formed of two walls that are spaced apart by a small distance, with at least one wall being transparent. The walls have electrodes thereon, usually in the form of transparent, electrically conductive coatings. The cell contains a light-modulating element (sometimes herein referred to as an “activatable material”), which may be either a liquid suspension of particles or a plastic film in which droplets of a liquid suspension of particles are distributed.
The liquid suspension (sometimes herein referred to as “a liquid light valve suspension” or as “a light valve suspension”) comprises small particles suspended in a liquid suspending medium. In the absence of an applied electrical field, the particles in the liquid suspension assume random positions due to Brownian movement. Hence, a beam of light passing into the cell is reflected, transmitted or absorbed depending upon the cell structure, the nature and concentration of the particles and the energy content of the light. The light valve is thus relatively dark in the OFF state. However, when an electric field is applied through the liquid light valve suspension in the light valve, the particles become aligned and for many suspensions most of the light can pass through the cell. The light valve is thus relatively transparent in the ON state.
For many applications it is preferable for the activatable material, i.e., the light modulating element, to be a plastic film rather than a liquid suspension. For example, in a light valve used as a variable light transmission window, a plastic film in which droplets of liquid suspension are distributed is preferable to a liquid suspension alone because hydrostatic pressure effects, e.g., bulging associated with a high column of liquid suspension, can be avoided through use of a film and the risk of possible leakage can also be avoided. Another advantage of using a plastic film is that, in a plastic film, the particles are generally present only within very small droplets and, hence, do not noticeably agglomerate when the film is repeatedly activated with a voltage.
A “light valve film” as used herein refers to a film having droplets of a liquid suspension of particles distributed in the film or in a portion of the film.
U.S. Pat. No. 5,409,734 exemplifies a type of light valve film that is made by phase separation from a homogeneous solution. Light valve films made by cross-linking emulsions are also known. See U.S. Pat. Nos. 5,463,491 and 5,463,492 both of which are assigned to the assignee of the present invention.
The following is a brief, non-limiting description of liquid light valve suspensions as known in the prior art.
A variety of liquid light valve suspensions are well-known in the art and such suspensions are readily formulated according to techniques well-known to one of ordinary skill therein. The term “liquid light valve suspension”, as noted above, when used herein means a “liquid suspending medium” in which a plurality of small particles are dispersed. The “liquid suspending medium” comprises one or more non-aqueous, electrically resistive liquids in which there is preferably dissolved at least one type of polymeric stabilizer which acts to reduce the tendency of the particles to agglomerate and to keep them dispersed and in suspension.
Liquid light valve suspensions useful in the present invention may include any of the so-called “prior art” liquid suspending media previously proposed for use in light valves for suspending the particles. Liquid suspending media known in the art which are useful herein include, but are not limited to, the liquid suspending media disclosed in U.S. Pat. Nos. 4,247,175, 4,407,565, 4,772,103, 5,409,734, 5,461,506 and 5,463,492. In particular, as described herein suspensions formed with the use of suspending media comprising polyalkyl (meth)acrylate and/or fluorinated (meth)acrylate suspending polymers are preferred for use in the present invention. In general one or both of the suspending medium or the polymeric stabilizer typically dissolved therein is chosen so as to maintain the suspended particles in gravitational equilibrium.
The polymeric stabilizer, when employed, can be a single type of solid polymer that bonds to the surface of the particles, but which also dissolves in the non-aqueous liquid(s) which comprise the liquid suspending medium. Alternatively, there may be two or more solid polymeric stabilizers serving as a polymeric stabilizer system. For example, the particles can be coated with a first type of solid polymeric stabilizer such as nitrocellulose which, in effect, provides a plain surface coating for the particles, together with one or more additional types of solid polymeric stabilizer that bond to or associate with the first type of solid polymeric stabilizer and which also dissolve in the liquid suspending medium to provide dispersion and steric protection for the particles. Also, liquid polymeric stabilizers may be used to advantage, especially in SPD light valve films, as described for example in U.S. Pat. No. 5,463,492.
Inorganic and organic particles may be used in a light valve suspension, and such particles may be either light-absorbing or light-reflecting in the visible portion of the electromagnetic spectrum.
Conventional SPD light valves have generally employed particles of colloidal size. As used herein the term “colloidal” means that the particles generally have a largest dimension averaging about 1 micron or less. Preferably, most polyhalide or non-polyhalide types of particles used or intended for use in an SPD light valve suspension will have a largest dimension which averages 0.3 micron or less and more preferably averages less than one-half of the wavelength of blue light, i.e., less than 2000 Angstroms to keep light scatter extremely low.
Notwithstanding the advantages which accrue due to the use of SPD films comprising a 1.455-1.463 refractive index (RI) siloxane matrix polymer as described in U.S. Pat. No. 6,416,827 B1 (“the '827 patent”) assigned to the assignee of the present invention, such films nonetheless remain subject to certain deficiencies. One particular area of concern with regard to such films is their performance capabilities in a wide variety of weather conditions (i.e., referred to herein as the film's “weatherability”). In particular, light valve films produced, according to the '827 patent, incorporating siloxane matrix polymers and polyalkyl (meth)acrylate and/or fluoroalkyl (meth)acrylate suspending polymers have been known to exhibit color changes, loss in light transmittance range and an increase in the off-state light transmission of the film upon prolonged exposure to accelerated weathering conditions applied to such films using an Atlas Ci 4000 Weather-Ometer (Atlas Electric Devices Company, Chicago, Ill.). Although not wishing to be bound by any particular theory, the inventors of the present invention have hypothesized that the phenyl content of the matrix polymer used in forming such films is insufficient in affording reasonable ultraviolet (UV) protection to the polyiodide crystals within the liquid light valve suspension, which crystals are inherently unstable to UV exposure under the conditions encountered in the Weather-Ometer, thus giving rise to the color change, loss of light transmittance range and an increase in off-state transmission as described above. It is known that phenyl groups strongly absorb in the ultraviolet region. It has therefore been determined that increasing the phenyl content of the matrix polymer, and thereby raising its refractive index (“RI”), as taught herein, will prevent the above-mentioned problems and lead to better performance by films produced therewith upon being subjected to a variety of weathering conditions.